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Mid-infrared Scanning Monochromator

IP.com Disclosure Number: IPCOM000247036D
Publication Date: 2016-Jul-28
Document File: 26 page(s) / 2M

Publishing Venue

The IP.com Prior Art Database

Abstract

Mid-infrared absorption spectroscopy is a promising technique to identify substances present in formation fluids and or drilling fluids. The identification is based on the spectral examination of unique absorption spectra of different substances present in the fluid mixtures. An optical instrument is required that is capable of analyzing the mid-infrared spectra. Because the absorption lines of hydrocarbons present are not significantly separated a high resolution spectrometer is necessary (Bandwidth: 15 nm at 3500 nm). A high resolution scanning monochromator is proposed as spectrometer. The monochromator is designed to have a simple mechanical setup to be less affected by component variations, regarding size and position, due to thermal expansion.

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Title:  Mid-infrared Scanning Monochromator

Abstract:  Mid-infrared absorption spectroscopy is a promising technique to identify substances present in formation fluids and or drilling fluids. The identification is based on the spectral examination of unique absorption spectra of different substances present in the fluid mixtures. An optical instrument is required that is capable of analyzing the mid-infrared spectra. Because the absorption lines of hydrocarbons present are not significantly separated a high resolution spectrometer is necessary (Bandwidth: 15 nm at 3500 nm).  A high resolution scanning monochromator is proposed as spectrometer. The monochromator is designed to have a simple mechanical setup to be less affected by component variations, regarding size and position, due to thermal expansion.

Description: 

Monochromator Design with Tilting Diffraction Grating

 

Required Spectral Range and Corresponding Diffraction Angle

Based on the grating equation the angle in which light with wavelength λ is diffracted can be calculated depending on the angle of incidence and diffraction order. Figure 1 shows the first and zeroth order  for a certain angle of incidence . Moreover, it shows the angle  , which is defined, as the angle between the incidence and the diffracted beam, working in first order.

Figure 1: Angles between incident light, grating normal, and diffraction orders for monochromator design

Considering the setup of a monochromator, the angle is constant because the grating is tilted to vary the angle of incidence and the wavelength analyzed by the detector. The mechanical setup is shown in Figure 3. is constant because of the mechanical mounting of the two mirrors placed before (collimating mirror) and after (focusing mirror) the grating. The position of both mirrors is fixed in the monochromator system and also the angle and their orientation to each other. Equations (1) show the mathematical correlation between the angle of incidence and the angle of diffraction, when the angle between the incident beam and the diffracted beam is fixed. Equation (2) shows the adapted grating equation.

      m=1,2,3,…

(1)

 

(2)

Equation (2) is used to calculate the corresponding wavelength to a certain angle of incidence. A grating with a grating frequency of 300 grooves/mm (Groove distance: d = 3,333.3 nm) is used for the calculations. The design also considers the zeroth order to not interfere with the measurement.  For m=0 no diffraction occur and is therefore not usable for the design of a spectrometer.

Figure 2 shows two diffraction orders depending on the angle of incidence and =25°. The wavelength range of interest is highlighted in Figure 2.

 

Figure 2: Corresponding wavelengths at several angles of incidence and diffraction orders for =25°; dashed lines mark areas of wavelengths and angles of incidence of interest.

According to the properties of a grating, a certain angle of incidence corresponds to several wavelengths...